1. Field of the Invention
The present invention relates to a color signal processing circuit of a camera's digital signal processing (hereinafter referred to as "DSP") chip for use with a color charge coupled device (hereinafter referred to as "CCD") or other kinds of solid image pickup devices, and more particularly to a color signal processing circuit for controlling the hue/gain upon color difference signals to reproduce original colors, and for performing frequency conversion to decrease the number of zoom data bits.
2. Description of the Prior Art
A word "hue" corresponds to one of three elements of color, and designates the distinct visual sensation with respect to a particular color's wavelength. Hue is a characteristic useful for distinguishing colors from one another such as red, green and blue.
Accordingly, once the color of an object picked up by a CCD is to be reproduced, for example on a monitor of a television or personal computer (hereinafter referred to as PC), a color signal must be multiplied by preset hue/gain coefficients to reproduce accurately the original color.
The conventional color signal processing circuit of a camera DSP chip having the conventional hue/gain control & frequency converting circuit includes, as shown in FIG. 1, a switch & clamp section 11 for rearranging color signals Wb (obtained by R+G+2B, i.e., white based on blue), Gr (obtained by R+2G, i.e., green based on red), Wr (obtained by 2R+G+B, i.e., white based on red) and Gb (obtained by B+2G, i.e., green based on blue) into a signal based on the blue set Gb and Wb and a signal based on the red set Gr and Wr. A Cr/Cb matrix section 12 performs mutual subtraction of the color signals Gb, Wb, Wr and Gr supplied from switch & clamp section 11 to provide the signals in the form of Cr (obtained by Wr-Gb) and Cb (obtained by Wb-Gr). An RGB matrix section 13 converts the Cr and Cb signals into the color signals in the form of R, G and B. Then, an rb-ROM 14 imposes gamma upon the R and B signals from RGB matrix section 13, and a g-ROM 15 imposes the gamma upon the G signal from RGB matrix section 13. A subtracting section 16 subtracts the G signal gamma-corrected in g-ROM 15 from the R and B signals gamma-corrected in rb-ROM 14 to form color difference signals in the form of R-G and B-G. Either the R-G and B-G signals from subtracting section 16 or zoom-processed RG and B-G signals from zoom-processing section 18 are synchronized with 4*(fsc) (where fsc denotes a color burst signal which is 3.58 MHz in case of the NTSC; National Television Systems Committee Standard) in a frequency converting section 17. The zoom-processing section 18 zoom-processes the R-G and B-G signals for 10 bits from subtracting section 16 upon selection of a zoom mode and supplies the result to frequency converting section 17. In a hue/gain controlling section 19 of the color signal processing circuit, the 10-bit R-G and B-G signals synchronized with 4*(fsc) in frequency converting section 17 are multiplied by hue/gain coefficients for conversion into B-Y and R-Y signals of 8 bits. Also, an encoder 20 loads the color burst signals onto the B-Y and R-Y signals from hue/gain controlling section 19 to provide the resultant video signals.
Here, zoom-processing section 18 which performs the digital zoom function may consist of a separate single chip or belong to the internal system application level of a PC.
The color signal processing circuit constructed as shown in FIG. 1 receives the output of the CCD and provides color difference signals B-Y and R-Y and the color burst signals, which constitute video signals suitable for the NTSC and PAL (Phase Alternation Line) standards.
In more detail, switch & clamp section 11 rearranges color signals Wb, Gr, Wr and Gb subjected to a color filter of the CCD into blue-tinged signals Gb and Wb and red-tinged signals Wr and Gr, and provides the rearranged signals to Cr/Cb matrix section 12.
Cr/Cb matrix section 12 executes the mutual substraction of the rearranged color signals Gb, Wb, Wr and Gr from switch & clamp section 11 to produce signals in the form of Cr and Cb. Then, RGB matrix section 13 utilizes the Cr and Cb signals to generate color signals in the form of R,G,B.
At this time, rb-ROM 14 imposes the gamma upon the R and B signals from RGB matrix section 13, and g-ROM 15 imposes the gamma upon the G signal from RGB matrix section 13.
Subtracting section 16 subtracts the green (G) signal gamma-corrected in g-ROM 15 from the red (R) and blue (B) signals gamma-corrected in rb-ROM 14 to produce the signals in the form of R-G and B-G.
That is, the R-G and B-G signals from subtracting section 16 as shown in FIG. 2C have 10 bits, which are synchronized with a main clock MCK shown in FIG. 2A. The main clock MCK is preferably a pixel clock of the CCD.
Main clock MCK is (8/3)fsc in case of a 250,000 pixel CCD, and 4fsc for 380,000 pixels. Here, fsc denotes the color subcarrier, which is 3.58 MHz for NTSC.
Since the color can be accurately reproduced only when the color signal has a phase corresponding to the color burst signal, the R-G and B-G signals should be synchronized with a clock CL (FIG. 2B) having a frequency four times as fast as the color burst signal fsc. This operation is performed in frequency converting section 17.
More specifically, frequency converting section 17 synchronizes the 10 bit R-G and B-G signals from subtracting section 16 with a clock CL(=4fsc) as shown in FIG. 2B to provide synchronized signals in the form shown in FIG. 2D. Here, upon shifting into the zoom mode, zoom-processing section 18 processes the 10 bit R-G and B-G signals and provides the processed signals to frequency converting section 17 which, in turn, synchronizes the 10 zoom-processed R-G and B-G bits with clock CL in the same manner as the ordinary unprocessed signals to arrange the signals in the order of R-G.sub.0, R-G.sub.0, B-G.sub.0, B-G.sub.1, R-G.sub.2, R-G.sub.2, . . . See FIG. 2D
In hue/gain controlling section 19, the R-G signal having the frequency and bit arrangement shown in FIG. 2D is multiplied by a R-hue coefficient and a R-gain coefficient. Then, the B-G signal is multiplied by a B-hue coefficient and a B-gain coefficient, and the R-G and B-G signals respectively multiplied by the different hue and gain coefficients are added to each other to form the B-Y and R-Y signals, thereby providing signals in the order of B-Y.sub.0, R-Y.sub.0, B-Y.sub.1. . . , as shown in FIG. 2E.
Here, the R-hue coefficient, R-gain coefficient, B-hue coefficient and B-gain coefficient are constants different from one another. The R-G signal multiplied by the R-hue coefficient is added to the B-G signal multiplied by the B-gain coefficient to form the B-Y signal having 8 bits. The R-G signal multiplied by the R-gain coefficient is added with the B-G signal multiplied by the B-hue coefficient to form the R-Y signal having 8 bits.
Thereafter, encoder 20 loads the color burst signals onto the B-Y and R-Y signals from hue/gain controlling section 19 to provide the color video signals suited to the NTSC and PAL standards.
Here, encoder 20 includes a frequency converting circuit, a phase shifting circuit for providing a phase shift with respect to the burst signal, and a burst signal generating circuit for converting the signal frequency into 5/4 times the speed of clock CL when in the PAL mode.
However, the above-described color signal processing circuit as shown in FIG. 1 has a drawback of increasing the external pins on the DSP chip because the zoom interface must communicate 10 bits (of the R-G and B-G signals) to zoom-processing section 19 when the latter is external to the DSP chip.
Furthermore, in the system application level, i.e., in reproducing the color of the CCD-sensed object onto the monitor of a PC, the R-G and B-G signals of 10 bits are utilized as the zoom data, thereby making the zoom processing laborious and difficult.